THR BRO Thermal Handbook 0110

8/12/2019 THR BRO Thermal Handbook 0110

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ThermoelectricHAND BOO K

Product Information

Assembly Information

Performance and Properties


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Table of Contents

Product Information

Introduction to Thermoelectrics ........................................................................................................................................................1

Structure and Function .....................................................................................................................................................................2

Temperature Control ........................................................................................................................................................................4

Parameters Required for Device Selection ........................................................................................................................................6

Sealant Options................................................................................................................................................................................7

Thermoelectric Arrays .......................................................................................................................................................................7

Design/Selection Checklist ...............................................................................................................................................................8

Thermoelectric Multistage (Cascade) Modules ................................................................................................................................9

Typical Device Performance. .............................................................................................................................................................9

Assembly Information

Assembly Tips .................................................................................................................................................................................10

Procedure For Assembling Lapped Modules To Heat Exchangers ...................................................................................................11

Procedure For Assembling Solderable Modules To Heat Exchangers ..............................................................................................12

Performance and Properties

Device Performance Formulae ........................................................................................................................................................13

Heat Transfer Formulae ..................................................................................................................................................................14Typical Properties of Materials (@ 21C) .......................................................................................................................................15

Reliability & Mean Time Between Failures (MTBF) .....................................................................................................................................16


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Introduction to ThermoelectricsSolid state heat pumps have been known since the discovery of

the Peltier effect in 1834. The devices became commercially avail-

able in the 60s with the development of advanced semiconductor

thermocouple materials in combination with ceramics substrates.

Thermoelectric modules (TEMs) are solid-state heat pumps that

require a heat exchanger to dissipate heat utilizing the Peltier Effect.During operation, DC current ows through the TEM to create heat

transfer and a temperature differential across the ceramic surfaces,

causing one side of the TEM to be cold, while the other side is hot.

A standard single-stage TEM can achieve temperature differentials

of up to 70C. However, modern growth and processing methods of

semiconductor materials are exceeding this limitation.

TEMs have several advantages over alternate cooling technologies.

They have no moving parts, so the solid state construction results in

high reliability. TEMs can cool devices down to well below ambient.

Colder temperatures can be achieved, down to minus 100C, by

using a multistage thermoelectric module in a vacuum environment.Thermoelectrics are able to heat and cool by simply reversing the

polarity, which changes the direction of heat transfer. This allows

temperature control to be very precise, where up to 0.01C can be

maintained under steady-state conditions. In heating mode TEMs

are much more efcient than conventional resistant heaters because

they generate heat from the input power supplied plus additional

heat generated by the heat pumping action that occurs.

A typical TEM measures 30 mm x 30 mm x 3.6 mm. Their geomet-

ric footprints are small as they vary from 2 x 2 mms to 62 x 62

mms and are light in weight. This makes thermoelectrics ideal for

applications with tight geometric space constraints or low weight

requirements when compared too much larger cooling technologies,

such as conventional compressor-based systems. TEMs can also be

used as a power generator converting waste heat into energy as

small DC power sources in remote locations.

When should you use thermoelectrics?

Thermoelectrics are ideal for applications that require active cooling

to below ambient and have cooling capacity requirements of up

to 600 Watts. A design engineer should consider them when the

system design criteria includes such factors as precise temperature

control, high reliability, compact geometry constraints, low weight

and environmental requirements. These products are ideal for many

of the consumer, food & beverage, medical, telecom, photonics and

industrial applications requiring thermal management.

ThermoelectricModules availablefrom Laird Technologies

CP Seriesoffer reliable cooling capacity in the range of 10 to 100

watts. They have a wide product breadth that is available in numer-

ous heat pumping capacities, geometric shapes, and input power

ranges. These modules are designed for higher current and larger

heat pumping applications with a maximum operating temperatureof 80C.

OptoTEC Serieshave a geometric footprint less than 13x13 mm

and are used in applications that have lower cooling requirements

of less than 10 watts. These modules offer several surface nishing

options, such as metallization or pre-tinning to allow for soldering

between TEM and mating conduction surfaces.

MS Seriesoffer the highest temperature differential, (T).

Each stage is stacked one on top of another, creating a multistage

module. Available in numerous temperature differentials and

geometric shapes, these modules are designed for lower heat

pumping applications.

ThermaTEC Seriesare designed to operate in thermal cycling

conditions that require reliable performance in both heating and

cooling mode (reverse polarity). Thermal stresses generated in these

applications will cause standard modules to fatigue over time. These

modules are designed for higher current and higher heat pumping

applications with a maximum operating temperature of 175C

UltraTEC Seriesoffer the highest heat pumping capacity within

a surface area. Heat pumping densities of up to 14 W/cm2, or twice

as high as standard modules, can be achieved. The cooling capacity

can range from 100 to 300 watts. TEMs are also ideal for applica-

tions that require low temperature differentials and high coefcientof performance (COP).


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Structure and FunctionSince thermoelectric cooling systems are most often compared

to conventional systems, perhaps the best way to show the

differences in the two refrigeration methods is to describe the

systems themselves.

A conventional cooling system contains three fundamental parts -the evaporator, compressor and condenser. The evaporator or cold

section is the part where the pressurized refrigerant is allowed to

expand, boil and evaporate. During this change of state from liquid

to gas, energy (heat) is absorbed. The compressor acts as the refrig-

erant pump and recompresses the gas to a liquid. The condenser

expels the heat absorbed at the evaporator plus the heat produced

during compression, into the environment or ambient.

A thermoelectric has analogous parts. At the cold junction, energy

(heat) is absorbed by electrons as they pass from a low energy

level in the p-type semiconductor element, to a higher energy level

in the n-type semiconductor element. The power supply provides

the energy to move the electrons through the system. At the hot

junction, energy is expelled to a heat sink as electrons move from a

high energy level element (n-type) to a lower energy level element

(p-type).

Thermoelectric Modules (TEMs) are heat pumps solid state devices

without moving parts, uids or gasses. The basic laws of thermody-

namics apply to these devices just as they do to conventional heat

pumps, absorption refrigerators and other devices involving the

transfer of heat energy.

An analogy often used to help comprehend a thermoelectric cool-ing system is that of a standard thermocouple used to measure

temperature. Thermocouples of this type are made by connecting

two wires of dissimilar metal, typically copper/constantan, in such

a manner so that two junctions are formed. One junction is kept at

some reference temperatures the other is attached to the control

device measurement. The system is used when the circuit is opened

at some point and the generated voltage is measured. Reversing

this train of thought, imagine a pair of xed junctions into which

electrical energy is applied causing one junction to become cold

while the other becomes hot.

Thermoelectric cooling couples (Fig. 1) are made from two elements

of semiconductor, primarily Bismuth Telluride, heavily doped to

create either an excess (n-type) or deciency (p-type) of electrons.

Heat absorbed at the cold junction is pumped to the hot junction at

a rate proportional to current passing through the circuit and the

number of couples.

In practical use, couples are combined in a module (Fig. 2) where

they are connected electrically in series, and thermally in parallel.

Normally a TEM is the smallest component commercially available.

TEMs are available in a great variety of sizes, shapes, operating

currents, operating voltages and ranges of heat pumping capacity.

The trend, however, is toward a larger number of couples operatingat lower currents. The user can select the quantity, size or capacity

of the module to t the exact requirement without paying for

excess power.

There is usually a need to use thermoelectrics instead of other

forms of cooling. The need may be a special consideration of size,

space, weight, efciency, reliability or environmental conditions

such as operating in a vacuum.

Once it has been decided that thermoelectrics are to be considered,

the next task is to select the thermoelectric(s) that will satisfy the

particular set of requirements. Three specic system parametersmust be determined before device selection can begin.

These are:

Tc Cold Surface Temperature

Th Hot Surface Temperature

Qc The amount of heat to be absorbed at the

Cold Surface of the TEM

Figure 1: Cross Section of a typical TE Couple

Figure 2: Typical TE Module Assembly


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In most cases, the cold surface temperature is usually given as part

of the problem that is to say that some object(s) is to be cooled to

some temperature. Generally, if the object to be cooled is in direct

intimate contact with the cold surface of the thermoelectric, the

desired temperature of the object can be considered the tempera-

ture of the cold surface of the TEM (Tc). There are situations where

the object to be cooled is not in intimate contact with the cold sur-face of the TEM, such as volume cooling where a heat exchanger is

required on the cold surface of the TEM. When this type of system is

employed, the cold surface of the TEM (Tc) may need to be several

degrees colder than the ultimate desired object temperature.

The Hot Surface Temperature is dened by two major parameters:

1) The temperature of the ambient environment to which

the heat is being rejected.

2) The efciency of the heat exchanger that is between the

hot surface of the TEM and the ambient environment.

These two temperatures (Tc & Th) and the difference betweenthem (T) are very important parameters and therefore must be

accurately determined if the design is to operate as desired.

Figure 3 represents a typical temperature prole across a

thermoelectric system.

The third and often most difcult parameter to accurately quantify is

the amount of heat to be removed or absorbed by the cold surface

of the TEM, (Qc). All thermal loads to the TEM must be considered.

These thermal loads include, but are not limited to, the active heat

load (I2R) from the electronic device to be cooled and passive heat

load where heat loss can occur through any object in contact with

ambient environment (i.e. electrical leads, insulation, air or gas sur-

rounding objects, mechanical fasteners, etc.). In some cases radiant

heat effects must also be considered.

Single stage thermoelectric modules are capable of producing a no

load temperature differential of approximately 70C. Temperature

differentials greater than this can be achieved by stacking one

thermoelectric on top of another. This practice is often referred to as

Cascading. The design of a cascaded device is much more complex

than that of a single stage device, and is beyond the scope of these

notes. Should a cascaded device be required, design assistance canbe provided by Laird Technologies Engineers.

Once the three basic parameters have been quantied, the selection

process for a particular module or array of TEMs may begin. Some

common heat transfer equations are attached for help in quantify-

ing Qc & Th.

There are many different modules or sets of modules that could

be used for any specic application. One additional criteria

that is often used to pick the best module(s) is Coefcient of

Performance (COP). COP is dened as the heat absorbed at the cold

junction, divided by the input power (Qc / P). The maximum COP

case has the advantages of minimum input power and therefore,

minimum total heat to be rejected by the heat exchanger (Qh =

Qc + P). These advantages come at a cost, which in this case is the

additional or larger TEM required to operate at COP maximum. It

naturally follows that the major advantage of the minimum COP

case is the lowest initial cost.Figure 3: Typical Temperature Relationship in a TEC


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Temperature ControlWhen designing a thermoelectric system power supplies, tempera-

ture controllers, and temperature sensors are components that also

require careful consideration.

Thermoelectric devices require a DC power source to operate. The

power supply output should be matched to the operational voltageof the thermoelectric modules and fans. Do not operate thermo-

electric devices above the specied maximum voltage. Doing so

will degrade the operational performance of the TEMs. The power

supply should also have a small ripple voltage (maximum of 10%

of full output). Ripple voltage is a uctuation of the power supply

output voltage and therefore is an AC component of the DC power

source. AC power will degrade the operational performance of the

TEMs. The degradation in performance due to ripple voltage can be

approximated by:

Temperature control can be accomplished by using one of two con-

trol methods: Open Loop (manual) and Closed Loop (automatic).

In the Open Loop method, an operator adjusts the output of the

power supply to achieve and maintain a steady temperature. In the

Closed Loop method an electronic controller runs an algorithm that

utilizes feedback data from sensors within the system to vary the

output of the power supply to control the temperature.

Temperature controllers can have a single directional output

or a bidirectional output. A temperature controller that has a

single directional output can operate in Heating or Cooling mode.

Controllers with a single directional output are used in maintaining

a constant temperature within a system surrounded by a

relatively constant ambient temperature (i.e. refrigeration or hot

food storage). A temperature controller with a bidirectional output

can operate in Heating and Cooling mode. Controllers with a

bidirectional output are used for maintaining a constant tempera-

ture within a system surrounded by an ambient environment with

large temperature uctuations (i.e. back-up battery storage, climate

control).

Temperature controllers can also have two regulation modes:

thermostatic (On/Off) or proportional control. Thermostatic control-

lers operate by turning on the TEM in order to heat or cool to a setpoint. The set point temperature tolerance is dened by a hysteresis

range. Once the set point is achieved the controller shuts off the

TEM. When the control temperature changes to outside the hyster-

esis range the controller turns on power to the TEMs and restarts

the cooling or heating mode process. This cycle continues until the

controller is shut down. Thermostatic control is often used in climate

control and refrigeration, where a narrow temperature swing can be

tolerated.

Proportional controllers use proportional regulation to maintain a

constant temperature with no swing in the control temperature.

This is often accomplished by using a Proportional Integral

Derivative (PID) algorithm to determine the output value and a

Pulse Width Modulation (PWM) output to handle the physical

control. When using a controller with a PWM output, a capacitor

can be placed (electrically) across the output to lter the voltage

to the TEM. Proportional controllers are often used in heating and

cooling systems where the temperature must stay constant (with

no change) regardless of the ambient temperature, such as liquid

chiller systems used in medical diagnostics.

Regardless of the controller used, the easiest feedback parameter

to detect and measure is temperature. The sensors most commonlyused by temperature controllers are thermocouples, thermistors,

and RTDs. Depending on the system; one or more temperature

sensors may be used for the purpose of control. The temperature

sensor feedback is compared by the controller to a set point or

another temperature to determine the power supply output.

The temperature feedback sensor(s) will most likely be determined

by the controller specied. Some controllers even include a sensor

with purchase.

T / Tmax = 1 / (1+N2), where N is a percentage of

current ripple, expressed as a decimal. Laird Technologies

recommends no more than a 10% ripple.


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To begin selection of a TEM controller,consider the following questions:

1. What is the maximum voltage & current of TEM used in

application? (also needed for selecting a power supply)

2. Does the system need to Heat, Cool or Heat & Cool?

3. Can the system tolerate a temperature swing of 3C?

Once answered, the selection of the basic functions of a tempera-

ture controller can be identied. The controller selected needs to be

capable of handling the maximum voltage and current to properly

control the TEM and power fans.

If the answers to question 2 is Heat or Cool and the answer to

question 3 is Yes then the required controller is single directional

and thermostatic.

If the answers to question 2 is Heat or Cool and the answer to

question 3 is No then the required controller is single directional

and proportional.

If the answers to question 2 is Heat & Cool and the answer to

question 3 is Yes then the required controller is bidirectional and

thermostatic.

If the answers to question 2 is Heat & Cool and the answer to

question 3 is No then the required controller is bidirectional and

proportional.

TEM controllers also can accommodate more advanced options to

trip alarms, control fan speeds and interface remotely with PC or UI,

but these are beyond the scope of this handbook. However, some

basic questions to consider for TEM controller designs are:

1. What alarms/indicators are required for User Interface?

2. Does the controller need to interface with a PC?

3. Does the TEM controller provide fan control?

4. Does the temperature set point need to be changed

by the end user?

Other design considerations may exist and should be considered

during system level design.

Laird Technologies offers a variety of Closed Loop Temperature

Controllers. The controller offering includes single and bidirectional

output controllers that employ thermistor temperature sensor

feedback, fan controls, alarms, and a range of control algorithms

ranging from thermostatic (ON/OFF) to PID. Laird Technologiesalso has the ability to customize and design temperature control-

lers to meet unique application requirements. Consult with a Laird

Technologies Sales Engineer on available product offerings or

customized solutions that may t to your design criteria.


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Parameters Required forDevice SelectionThere are certain minimum specications that everyone must

answer before the selection of a thermoelectric module (TEM) can

begin. Specically there are three parameters that are required.

Two of these parameters are the temperatures that dene thegradient across the TEM. The third parameter is the total amount

of heat that must be pumped by the device.

The temperature gradient across the TEM, actual T is not the same

as the apparent, system level T. The difference between these two

Ts is often ignored, which results in an under-designed system.

The magnitude of the difference in Ts is largely dependent on the

thermal resistance of the heat exchangers that are used on the hot

or cold sides of the TEM.

Unfortunately, there are no Hard Rules that will accurately

dene these differences. Typical allowances for the hot side of

a system are:

1. nned forced air: 10 to 15C

2. free convection: 20 to 40C

3. liquid exchangers: 2 to 5C above liquid temperature

Since the heat ux densities on the cold side of the system are

considerably lower than those on the hot side, an allowance of

about 50% of the hot side gures (assuming similar types of heat

exchangers) can be used. It is good practice, to check the outputs

of the selection process to reassure that the heat sink design

parameters are reasonable.

The third parameter that must be identied for the selection pro-

cess, is the total heat to be pumped by the TEM. This is often the

most difcult number to estimate. To reduce the temperature

of an object, heat must be removed faster than heat enters it.

There are generally two broad classications of the heat that

must be removed from the device. The rst is the real, sensible or

active heat load. This is the load that is representative of what

wants to be done. This load could be the I2R load of an electrical

component, the load of dehumidifying air, or the load of cooling

objects. The other kind of load is often referred to as the passive

heat load. This is the load due to the fact that the object is cooler

than the surrounding environment. This load can be composed of

conduction and convection of the surrounding gas, leak through

insulation, conduction through wires, condensation of water, and in

some cases formation of ice. Regardless of the source of these pas-sive loads, they must not be ignored.

There are other things that may be very important to a specic

application, such as physical dimensions, input power limitations

or cost. Even though these are important, they are only secondary.

Laird Technologies approach to thermoelectric module selection/

recommendation utilizes a proprietary computer aided design

program called AZTECwhich selects an optimized thermoelec-

tric design from a given set of parameters: hot side temperature,

desired cold side temperature, and the total heat load to be pumped

over the actual T.

A checklist has been enclosed to assist with dening your appli-

cations existing conditions. If you should require any further

assistance please contact one of Laird Technologies sales engineers.


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Sealant OptionsMost applications operate in a room temperature environment

and cool to below dew point. As a result, moisture in the environ-

ment will condense onto the cold side heat exchanger and may

accumulate around mounting hardware and eventually penetrate

to the TEM. The presence of moisture will cause corrosion that willdegrade the useful life of a thermoelectric. Two perimeter seal-

ants are generally used because they provide moisture protection

against condensation, have high dielectric strength and low thermal

conductivity.

Silicone (RTV) is an all purpose sealant that exhibits good sealing

characteristics and retains its elastomeric properties over a wide

temperature range, -60 to 200C. The sealant is non-corrosive to

many chemicals and exhibits good electrical properties with low

thermal conductivity. It is suitable for high volume applications for

ease of use and is cost effective. However, over time it is impervious

to vapor migration that can actually trap small amounts of moistureinside the TEM once the vapor condenses. This may or may not be a

problem dependent on life expectancy of application and environ-

mental conditions.

Epoxy (EP) is an effective barrier to moisture that exhibits a useable

temperature range of -40 to 130C. When cured the material is

completely uni-cellular and therefore the moisture absorption is

negligible. The material exhibits a low dielectric constant, low coef-

cient of thermal expansion and low shrinkage. Epoxies are ideal

for applications requiring long life expectancies. However, applying

epoxy onto TEM can be cumbersome as multiple llers are required

to be mixed and working life tends to be short, which makes it more

difcult to automate for higher volume production runs.

It should be noted that since sealants come in contact with the top

and bottom ceramic, they act as a thermal paths and transfer heat.

The thermal conductivity of RTV and Epoxy is low, but it still can

diminish the cooling performance of a TEM by up to 10%. However,

it is necessary to specify for applications that maybe susceptible to

condensation.

Thermoelectric ArrayWiring multiple TEMs together is commonly referred to as a TE

array. The decision to wire TEMs in series or in parallel is primarily

based on available input power requirements. No additional per-

formance benet will be achieved by wire arrangement. TE arrays

are commonly used for higher heat pumping capacities and can be

more efcient than a single TEM by taking advantage of dissipating

heat over a larger surface area. When mounting a TE array onto a

heat exchanger, the recommended lapping tolerances are 0.025

mm for two TEMs and 0.0125mm for three or more. This is done

to maximize the thermal contact between the TEM and mating heat

exchangers.

One advantage of wiring a TE array in parallel versus in series is

that the entire TE array will not fail if one TEM has an open circuit.

This can be benecial for applications that require redundancy.


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Design/Selection ChecklistThe information requested below is vital to the design/selection of a thermoelectric device to achieve your desired performance.

Please attempt to dene as many of your applications existing conditions and limiting factors as possible.

(Please indicate units on all parameters.)

I. Ambient EnvironmentTemperature = ____________________

o Air

o Vacuum

o Other

II. Cold SpotTemperature: ______________

Size: ______________

Insulated? ___________Type:_____________Thickness: _____________

Desired Interface:

o Plate

o Fins

o Fluid Flow (parameters) ________________

o Other _______________

III. Heat Sinko Finned - Free Convection

o Finned - Forced Convection

o Liquid Cooled

Maximum Heat Sink Temp. _________________ -or-Heat Sink Rating (C/W) ___________________

IV. Heat Load at Cold Spot = ____________________(if applicable, above should include:)

Active:

I2R __________________

Passive:

Radiation= _________________

Convection= ________________

Insulation Losses= _________________

Conduction Losses= ________________(e.g. leads)

Transient Load= _________________(Mass - time)

V. Restrictions on Power Available (indicate most important)o Current: _________________

o Voltage: __________________

o Power: __________________

o No Restrictions

VI. Restrictions on Size: ___________________

VII. To ensure the most effective response:

Please provide a rough, dimensioned sketch of the application, indicating the

anticipated physical conguration and thermoelectric module placement.

Please print this form and ll in the blanksTelephone: 888 246-9050 Email: [email protected]


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Thermoelectric Multistage(Cascade) ModulesA multistage thermoelectric module should be used only when a

single stage module does not meet control temperature require-

ments. Figure 4 depicts two graphs: the rst shows the T vs.

Normalized Power input (Pin/Pmax) of single and multistage mod-ules. The second graphs shows the T vs. COP. COP is dened as

the amount of heat absorbed at the cold side of the TEM (in thermal

watts) divided by the input power (in electrical watts).

These gures should help identify when to consider cascades since

they portray the effectiveT range of the various stages. A two-

stage cascade should be considered somewhere between a T of

40C and 65C. Below a T of 40C, a single stage module may

be used, and a T above 65C may require a 3, 4 or even 5 stage

module.

Figure 4: Multistage TemperatureDifferential Graphs

There is another very signicant factor that must always be con-

sidered and that is cost. As the number of stages increase, so doesthe cost. Certain applications require a trade-off between COP and

cost. As with any other thermoelectric system, to begin the selection

process requires the denition of at least three parameters:

Tc Cold Side Temperature

Th Hot Side Temperature

Qc The amount of heat to be removed

(absorbed by the cooled surface of the TEM) (in watts)

Once T (Th - Tc) and the heat load have been dened, utilization of

Figure 4 will yield the number of stages that should be considered.

Knowing COP and Qc , input power can also be estimated. The

values listed in Figure 4 are theoretical maximums. Any device that

is actually manufactured will rarely achieve these maximums, but

should closely approach this value.

Laird Technologies offers a line of MS Series cascades though thereare no standard applications. Each need for a cascade is unique, so

too should be the device selected to ll the need. Laird Technologies

has developed a proprietary computer aided design selection tool

called Aztecto help select a device. The three parameters listed

are used as inputs to the programs. Other variables such as physical

size, and operating voltage or current can, within limits, be used to

make the nal selection. More than 40,000 different cascades can

be assembled utilizing available ceramic patterns. This allows near

custom design, at near standard prices. When the three param-

eters have been dened, please contact a Laird Technologies sales

engineer for assistance in cascade selection.

Typical Device PerformanceWhen PERFORMANCE vs. INPUT POWER is plotted for any thermo-

electric device, the resultant curve will appear as in gure 5 below.

Performance can be T (Th - Tc), heat pumped at the cold side (Qc ),

or as in most cases, a combination of these two parameters.

Input power can be current (I), voltage (V) or the product of IV.

When we refer to the Tmax or Qc max, we are referring to that

point where the curve peaks. The same is true when referring to

either Imax or Vmax. Since operating at or very near the peak is

relatively inefcient, most devices are operated somewhere

between 40% and 80% of Input Power MAX.

As stated, devices are normally operated on the near-linear,

upward sloping portion of the curve. When automatic or closed loop

temperature control is being used, current or voltage limits should

be set below the MAX intercepts.

Figure 5: Performance vs Input Power


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Assembly TipsThe techniques used in the assembly of a thermoelectric system

can be as important as the selection of the thermoelectric module

(TEM). It is imperative to keep in mind the purpose of the assembly

namely to transfer heat. Generally a TEM, in cooling mode, moves

heat from an object to ambient environment. All of the mechanical

interfaces between the device to be cooled and ambient are alsothermal interfaces. Similarly all thermal interfaces tend to inhibit the

transfer of heat or add thermal resistance to system, which lowers

COP. Again, when considering assembly techniques every reason-

able effort should be made to minimize the thermal resistance

between hot and cold surfaces.

Mechanical tolerances for heat exchanger surfaces should not

exceed .025 mm/mm with a maximum of .076 mm total Indicated

Reading. If it is necessary to use multiple TEMs in an array between

common plates, then the height variation between modules should

not exceed 0.025 mm (request tolerance lapped modules when

placing order). Most thermoelectric assemblies (TEAs) utilizethermal interface materials, such as grease. The grease thickness

should be kept to 0.025 .013 mm to minimize thermal resistance.

A printers ink roller and screen works well for maintaining grease

thickness. When these types of tolerances are to be held, a certain

level of cleanliness must be maintained to minimize contaminants.

Once the TEMs have been assembled between the heat exchangers,

some form of insulation should be used between the exchangers

surrounding the modules. Since the area within the module, (i.e. the

element matrix), is an open DC circuit and a temperature gradient

is present, air ow should be minimized to prevent condensation.

Typically, a TEM is about 5.0 mm thick, so any insulation that can

be provided will minimize heat loss between hot and cold side heat

exchangers. The presence of the insulation/seal also offers protec-

tion from outside contaminants.

The insulation/seal is often most easily provided by inserting a die

cut closed cell polyurethane foam around the cavity and sealing

with either an RTV type substance or, for more physical integrity, an

epoxy coat. Whatever form is used, it should provide the protection

outlined above. It is often desirable to provide strain relief for the

input lead wires to TEM, not only to protect the leads themselves,

but to help maintain the integrity of the seal about the modules.

We have included an Assembly Tips drawing (Fig. 6). This drawing

shows the details of the recommended construction of a typical

assembly. The use of a spacer block yields maximum heat trans-

fer, while separating the hottest and coldest parts of the system,

by the maximum amount of insulation. The spacer blocks are

used on the cold side of the system due to the lower heat ux

density. In addition, the details of a feed thru and vapor sealing

system that can be used for maximum protection from the

environment are shown.

If you follow the recommendations shown in these drawings than

you will see a signicant improvement in performance. When

testing an assembly of this type it is important to monitor tempera-

ture. Measuring temperature of the cooling uids, inlet and outlet

temperatures as well as ow rates is necessary. This is true if either

gas or liquid uids are used. Knowing input power to the TEM, both

voltage and current, will also help in determining the cause of a

potential problem.

In addition we have enclosed step-by-step procedure for assem-

bling Laird Technologies modules, Solderable or Lapped modules to

heat-exchangers.

If you should require any further assistance, please

contact one of our engineers. Our many years of

experience in working with customers ensuring reli-

able and efcient application of our products hasproven to be essential to product success.

Figure 6: Assembly Tips Drawing

Figure 7: Assembly Procedures Drawing


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Asia: +86.755.2714.1166 1

Device Performance Formulae

Heat Pumped at Cold Surface: QC= 2N [aI T

C- ((I2r) / (2 G)) -kT G]

Voltage: V = 2N [((I r) / G) + (aT)]

Maximum Current: Imax

=(kG / a) [(1 + (2 Z TH)]1/2- 1]

Optimum Current: Iopt

= [kT G (1 + (1 + Z Tave

)1/2)] / (aTave

)

Optimum COP (calculated at Iopt

): COPopt

=(Tave

/ T) [((1 + Z Tave

)1/2 - 1) / ((1 + Z Tave

)1/2+ 1)] - 1/2

Maximum T with Q = 0 Tmax

= Th - [(1 + 2 Z TH)1/2- 1) / Z]

Notation Denition

TH Hot Side Temperature (Kelvin)

TC Cold Side Temperature (Kelvin)

T TH- T

C(Kelvin)

Tave

1/2 (TH+ T

C) (Kelvin)

G Area / Length of T.E. Element (cm)

N Number of Thermocouples

I Current (amps)

COP Coefcient of Performance (QC

/ IV)

a Seebeck Coefcient (volts / Kelvin)

r Resistivity (cm)

k Thermal Conductivity(watt / (cm Kelvin))

Z Figure of Merit(a2/ (rk)) (Kelvin-1)

S Device Seebeck Voltage(2 aN) (volts / Kelvin)

R Device Electrical Resistance(2 rN / G) (ohms)

K Device Thermal Conductance(2 kN G) (Watt / Kelvin)

Geometry Factor (G)

TEM G TEM G

OT 08 -xx- 05 0.016OT 12 -xx- 06 0.024

OT 15 -xx- 05 0.030

OT 20 -xx- 04 0.040

CP 08 -xx- 06 0.042

CP 08 -xx- 05 0.052

CP 10 -xx- 08 0.050

CP 10 -xx- 06 0.061

CP 10 -xx- 05 0.079

CP 14 -xx- 10 0.077

CP 14 -xx- 06 0.118

CP 14 -xx- 045 0.171

CP 20 -xx- 10 0.184

CP 20 -xx- 06 0.282

CP 28 -xx- 06 0.473

CP 5 -xx- 10 0.778CP 5 -xx- 06 1.196

PT 2 -12- 30 0.046

PT 3 -12- 30 0.057

PT 4 -12- 30 0.079

PT 4 -7- 30 0.076

PT 4 -12- 40 0.076

PT 6 -xx- xx 0.121

PT 8 -xx- xx 0.171

HT 2 -12- 30 0.046

HT 3 -12- 30 0.057

HT 4 -12- 30 0.079

HT 4 -7- 30 0.076

HT 4 -12- 40 0.076

HT 6 -xx- xx 0.121

Typical Material Parameters

T (Kelvin) r k Z 273 9.2 x 10-4 1.61 x 10-2 2.54 x 10-3

300 1.01 x 10-3 1.51 x 10-2 2.68 x 10-3

325 1.15 x 10-3 1.53 x 10-2 2.44 x 10-3

350 1.28 x 10-3 1.55 x 10-2 2.22 x 10-3

375 1.37 x 10-3 1.58 x 10-2 1.85 x 10-3

400 1.48 x 10-3 1.63 x 10-2 1.59 x 10-3

425 1.58 x 10-3 1.73 x 10-2 1.32 x 10-3

450 1.68 x 10-3 1.88 x 10-2 1.08 x 10-3

475 1.76 x 10-3 2.09 x 10-2 8.7 x 10-4

These tables and attributes are also available on AZTECthermoelectric module selection software


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14 Asia: +86.755.2714.1166

Heat Transfer FormulaeNOTE:Due to the relatively complex nature of heat transfer, results gained from application of these formulae, while useful,

must be treated as approximations only. Design safety margins should be considered before nal selection of any device.

1) Heat gained or lost through the walls of an insulated container:

Where:Q = Heat (Watts)

A = External surface area of container (m2)

T = Temp. difference (inside vs. outside of container) (Kelvin)

K = Thermal conductivity of insulation (Watt / meter Kelvin)

X = Insulation thickness (m)

2) Time required to change the temperature of an object:

Where:t = Time interval (seconds)

m = Weight of the object (kg)

Cp= Specic heat of material (J / (kg K))

T = Temperature change of object (Kelvin)

Q = Heat added or removed (Watts)

NOTE:It should be remembered that thermoelectric devices do not add or remove heat at a constant rate when T is

changing. An approximation for average Q is:

3) Heat transferred to or from a surface by convection:

Where:Q = Heat (Watts)

h = Heat transfer coefcient (W / (m2K))

(1 to 30 = Free convection - gases, 10 to 100 = Forced convection - gases)

A = Exposed surface area (m2)

T = Surface Temperature - Ambient (Kelvin)

Conversions:

Thermal Conductivity 1 BTU / hr ft F = 1.73 W / m K 1 W / m K = 0.578 BTU / hr ft F

Power (heat ow rate) 1 W = 3.412 BTU / hr 1 BTU / hr = 0.293 W

Area 1 ft2= 0.093 m2

1 m2= 10.76 ft2

Length 1 ft = 0.305 m 1 m = 3.28 ft

Q = (A x T x K) / (X)

t = (m x Cp x T) / Q

Q = h x A x T

Qave = (Q (Tmax) + Q (Tmin)) / 2

Specic Heat 1 BTU / lb F = 4184 J / kg K 1 J / kg K = 2.39 x 10-4 BTU / lb F

Heat Transfer Coefcient 1 BTU / hr ft2F = 5.677 W / m2K 1 W / m2K = 0.176 BTU / hr ft2F

Mass 1 lb = 0.4536 kg 1 kg = 2.205 lb


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Asia: +86.755.2714.1166 1

Air 1.2 0.026 1004

Alumina Ceramic-96% 3570 35.3 837 6.5

Aluminum Nitride Ceramic 3300 170-230 920 4.5

Aluminum 2710 204 900 22.5

Argon (Gas) 1.66 0.016 518

Bakelite 1280 0.23 1590 22.0

Beryllia Ceramic-99% 2880 230 1088 5.9

Bismuth Telluride 7530 1.5 544 13.0

Brass 8490 111 343 18.0

Bronze 8150 64 435 18.0

Concrete 2880 1.09 653 14.4

Constantan 8390 22.5 410 16.9

Copper 8960 386 385 16.7

Copper Tungsten 15650 180-200 385 6.5

Diamond 3 500 2300 509

Ethylene Glycol 1116 0.242 2385

Glass (Common) 2580 0.80 795 7

Glass Wool 200 0.040 670

Gold 1 9320 310 126 14.2

Graphite 1625 25-470 770 4.7

Iron (Cast) 7210 83 460 10.4

Kovar 8360 16.6 460 5.0

Lead 11210 35 130 29.3

Molybdenum 10240 142 251 4.9

Nickel 8910 90 448 11.9

Nitrogen (Gas) 1.14 0.026 1046

Platinum 21450 70.9 133 9.0

Plexiglass (Acrylic) 1410 0.26 1448 74

Polyurethane Foam 29 0.035 1130

Rubber 960 0.16 2009 72

Silicone (Undoped) 2330 144 712

Silver 10500 430 235

Solder (Tin/Lead) 9290 48 167 24.1

Stainless Steel 8010 1 3.8 460 17.1

Steel (Low Carbon) 7850 48 460 11.5

Styrofoam 29-56 .029 1.22 Teon 2200 0.35

Thermal Grease 2400 0.87 2093

Tin 7310 64 226 23.4

Titanium 4372 20.7 460 8.2

Water (@ 70F) 1000 0.61 4186

Wood (Oak) 610 0.15 2386 4.9

Wood (Pine) 510 0.11 2805 5.4

Zinc 7150 112 381 3 2.4

Material Density Thermal Specic Thermal Expansion

Name kg/m3 Conductivity Heat Coefcient x 10-6

W/m-K J/kg-K cm/cm/C

Typical Properties of Materials (@ 21C)


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ericas: +1.888.246.9050 www.lairdtech.comope: +46.31.704.67.57

a: +86.755.2714.1166 16

Reliability & Mean TimeBetween Failures (MTBF)Thermoelectric devices are highly reliable due to their solid state

construction. Although reliability is application dependent, MTBFs

calculated as a result of tests performed by various customers

are on the order of 200,000 hours at room temperature. Elevatedtemperature (80C) MTBFs are conservatively reported to be on the

order of 100,000 hours. Field experience by hundreds of customers

representing more than 7,500,000 of our CP type modules and more

than 800,000 OptoTEC type modules during the last ten years

have resulted in a failure return of less than 0.1%. More than 90%

of all modules returned were found to be failures resulting from

mechanical abuse or overheating on the part of the customer.

Thus, less than one failure per 10,000 modules used in systems

could be suspect of product defect. Therefore, the combination

of proper handling, and proper assembly techniques will yield an

extremely reliable system.

Historical failure analysis has generally shown the cause of failure

as one of two types: Mechanical damage as a result of improper

handling or system assembly techniques.

Moisture:

Moisture must not penetrate into the thermoelectric module area.

The presence of moisture will cause an electro-corrosion that

will degrade the thermoelectric material, conductors and solders.

Moisture can also provide an electrical path to ground causing an

electrical short or hot side to cold side thermal short. A proper seal-

ing method or dry atmosphere can eliminate these problems.

Shock and Vibration:

Thermoelectric modules in various types of assemblies have for

years been used in different Military/Aerospace applications.

Thermoelectric devices have been successfully subjected to shock

and vibration requirements for aircraft, ordinance, space vehicles,

shipboard use and most other such systems. While a thermoelectric

device is quite strong in both tension and compression, it tends to

be relatively weak in shear. When in a severe shock or vibration

environment, care should be taken in the design of the assembly to

ensure compressive loading of thermoelectric modules.

Mechanical Mounting:

A common failure mode during assembly of a thermoelectric

module is un-even loading induced by improper torqing,

bolting patterns, and mechanical conditions of heat exchangers.

The polycrystalline thermoelectric material exhibits less strength

perpendicular to the length (growth axis) than the horizontal axis.

Thus, the thermoelectric elements are quite strong in compres-

sive strength and tend to be weak in the shear direction. During

assembly, un-even torquing or un-at heat exchangers can cause

severe shear forces. (See assembly instructions for proper mounting

techniques.)

Inadvertent Overheating of the Module:

The direct soldering process does result in temperature restriction

for operation or storage of the modules.

At temperatures above 80C two phenomena seriously reduce

useful life:

Above 80C copper diffusion into the thermoelements occurs due

to increasing solid solubility in the thermoelectric material and

increasing diffusion rate. At 100 - 110C the combined solubility

and diffusion rate could result in approximately 25% loss of device

performance within 100 hours.

Above 85C in the soldering process (using Bismuth-Tin Alloy) smal

amounts of selenium, tellurium, antimony and nickel are inherently

dissolved into the bismuth-tin solder. Although the melting point

of the base solder is 136C, the combined mixture of all elements

results in either a minute eutectic phase or a highly effective solid

state reaction occurring at above 85C that starts to delaminatethe ends of the thermoelements by physical penetration between

cleavage planes in the thermoelectric material. This results in a

mechanical failure of the interface.


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THR BRO Thermal Handbook 0110

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